In optical scanning systems, such as xerographic copiers, it is often necessary to project an array of light beams onto an image plane. For maximum image resolution the individual spots of light projected onto the image plane should be tightly defined and for maximum signal to noise performance any cross talk between the light beams should be controlled precisely.
Apertured emitters, shown in FIG. 1, have been used in the prior art to limit somewhat the cross talk between individual light beams and to define the shape and size of the resultant light spots. Unfortunately, simple apertures severely limit the intensity of the projected light spot since all incoming light beyond the apertures is cut off.
Total internal reflection light guides, such as the light pipes available from Collimated Holes, Inc. and shown in FIG. 2, have also been used in the prior art to project arrays of light spots. The light pipe walls are made from a plastic having a refractive index which is less than the refractive index of the glass light pipe channels (n.sub.1 &gt;n.sub.2). Air is not a suitable channel material since the refractive index of air is approximately equal to 1 and selection of a realizable wall material would not be possible. Use of glass channels increases manufacturing complexity and light pipe cost. In order to minimize reflection at the bottom of the channels, the emitters are encapsulated in silicone which further increases light pipe cost while decreasing reliability.
Graded refractive index rod lenses, such as those manufactured by Nippon Sheet Glass Company, Ltd., provide improved light spot shaping and sizing. Unfortunately, these lenses are very expensive and have poor optical efficiency relative to light pipes.
In accordance with the illustrated preferred embodiment of the present invention an inexpensive array of limited internally reflective light guides projects an array of tightly defined light spots with minimum cross talk. The walls of the light guides are optically opaque and the wall surfaces have an index of refraction which is greater than the index of refraction of the guide channels (n.sub.1 &lt;n.sub.2). Light which hits a wall surface at an angle between 90.degree. (from the normal relative to the wall) and Brewster's angle tends to be reflected and propagated through the light guide channel toward the image plane. Light which hits a wall surface at an angle from the normal which is less than Brewster's angle tends to be refracted into the wall where it is absorbed since reflectivity at such an angle is low. Thus, cross talk due to light entering an undesired adjacent channel is reduced without attenuation of the desired light beam and spot definition may be tightly controlled since the limited reflectivity of the walls angularly constrains the exiting light beam. In addition, since an air channel is possible, manufacturing costs are minimized and reliability is increased.